Advance Information MC68HC908MR16/MC68HC908MR32 — Rev. 4.0
116 Clock Generator Module (CGM) MOTOROLA
Clock Generator Module (CGM)
These conditions apply when the PLL is in automatic bandwidth control
mode:
•The ACQ
bit (see 8.6.2 PLL Bandwidth Control Register) is a
read-only indicator of the mode of the filter. For more information,
see 8.4.2.2 Acquisition and Tracking Modes.
•The ACQ
bit is set when the VCO frequency is within a certain
tolerance, ∆TRK, and is cleared when the VCO frequency i s out of
a certain tolerance, ∆UNT. For more information, see
8.9 Acquisition/Lock Time Specifications.
• The LOCK bit is a read-only indicator of the locked state of the
PLL.
• The LOCK bit is set when the VCO frequency is wit hin a certain
tolerance, ∆Lock, and is cleared when the VCO frequency is out of
a certain tolerance, ∆UNL. For more information, see
8.9 Acquisition/Lock Time Specifications.
• CPU interrupts can occur if enabled (PLLIE = 1) when the PLL’s
lock condition changes, toggling the LOCK bit. For more
information, see 8.6.1 PLL Control Register.
The PLL also may operate in manual mode (AUTO = 0). Manual mode
is used by systems that do not require an indicator of the lock condition
for proper operation. Such systems typically operate well below fBUSMAX
and require fast startup. These conditions apply when in manual mode:
•ACQ is a writable control bit that controls the mode of the filter.
Before turning on the PLL in manual mode, the ACQ bit must be
clear.
• Before entering tracking mode (ACQ = 1), software must wait a
given time, tACQ (see 8.9 Acquisition/Lock Time
Specifications), after turning on the PLL by setting P LLON in th e
PLL control register (PCTL).
• Software must wait a given time, tAL, after entering tracking mode
before selecting the PLL as the clock source to CGMOUT
(BCS = 1).
• The LOCK bit is disabled.
• CPU interrupts from the CGM are disabled.